Fuel cell system

ABSTRACT

A fuel cell system is provided, the system comprising: a fuel cell ( 1 ) configured to generate electric power by use of a fuel gas; a reformer ( 2 ) configured to generate the fuel gas by reforming a raw material; a combustion burner ( 2   a ) configured to supply heat for reforming; a raw material feeder ( 10 ) configured to control the feed rate of combustion fuel to the combustion burner; a combustion fan ( 2   b ) configured to supply air to the combustion burner; a fuel gas passage (R 1 , R 4 ); an off gas passage (R 3 , R 5 ); a bypass passage (R 2 ); a switching valve ( 8 ); and a controller ( 101 ), wherein the inside of the fuel cell is filled with the raw material before the fuel gas is supplied, and wherein the controller controls the raw material feeder so as to reduce the feed rate of the combustion fuel to the combustion burner when controlling the switching valve so as to shut off the bypass passage to supply the fuel gas generated in the reformer to the fuel cell. This fuel cell system is environmentally-friendly and capable of suppressing the emission of carbon monoxide at the start of a power generating operation to mitigate the adverse effect upon the ecosystem.

TECHNICAL FIELD

The present invention relates to a fuel cell system for generatingelectric power by use of hydrogen and oxygen. More particularly, thepresent invention relates to a fuel cell system that utilizes thecombustion heat of flammable material to generate hydrogen from rawmaterial and uses this hydrogen for power generation.

BACKGROUND ART

In fuel cell systems capable of high-efficiency, small-scale powergeneration, it is easy to construct a system for utilizing heat energygenerated during power generation. Thanks to this, fuel cell systemshave heretofore been developed as a dispersed power generation systemcapable of achievement of high energy utilization efficiency.

Fuel cell systems have fuel cell as the main body of the powergeneration section. The fuel cell directly converts the chemical energyof fuel gas and oxidizing gas into electric energy through apredetermined electrochemical reaction. Therefore, in the fuel cellsystems, the fuel gas and oxidizing gas are respectively supplied to thefuel cell during power generating operation. Then, in the fuel cell, thespecified electrochemical reaction which uses the supplied fuel gas andoxidizing gas proceeds, thereby generating electric energy. The electricenergy generated in the fuel cell is fed to loads from the fuel cellsystem. The fuel cell systems discussed herein generally include areformer and a blower. In the reformer, the hydrogen-rich fuel gas isgenerated by the steam reforming reaction which uses water and a rawmaterial containing an organic compound composed of at least carbon andhydrogen, which is a natural gas. This fuel gas is supplied to the fuelcell as a fuel for power generation. It should be noted that the steamreforming reaction proceeds with the reforming catalyst of the reformerbeing heated by, for example, a combustion burner. The blower suctionsair from the atmosphere. This air is fed to the fuel cell as theoxidizing gas used for power generation.

In a heretofore known fuel cell system, when stopping a power generatingoperation, the supply of the raw material to the reformer is cut off.This stops the supply of the fuel gas from the reformer to the fuelcell, stopping the progress of the electrochemical reaction in the fuelcell. As a result, the supply of electric power from the fuel cellsystem to the loads is stopped. If the supply of the raw material to thereformer is cut off, the fuel gas, which has been generated before thecutoff of the raw material, will stagnate in the fuel cell and itssurrounding area throughout the period during which the power generatingoperation is stopped. In this case, if the stagnant fuel gas is mixedwith air coming from the combustion burner opened to the atmosphereowing to natural convection, hydrogen contained in the fuel gas will berapidly oxidized by oxygen contained in the air, which gives rise to arisk that the fuel cell system may be damaged by the reaction heatgenerated by the oxidation reaction.

To prevent the stagnation of the fuel gas within the fuel cell system,the known fuel cell system is configured to feed inert gas such asnitrogen gas to the passage where the fuel gas is stagnant during apower generation stop period and to burn the fuel gas forced out of thepassage in a combustion burner. According to this configuration, thestagnation of the fuel gas within the fuel cell etc. during a powergeneration stop period can be prevented and therefore the rapidoxidation of hydrogen contained in the fuel gas can be prevented so thatthe fuel cell system ensures high security.

This known fuel system, however, requires an inert gas feeding meanssuch as a nitrogen cylinder installed within or near the fuel cellsystem in order to replace the stagnant fuel gas with the inert gas suchas nitrogen gas. This makes the fuel cell system large-sized andtherefore makes it difficult to use the fuel cell system as a stationarydispersed power generation system for household use or an electricalvehicular power plant. Furthermore, the use of the feeing means of theinert gas such as nitrogen gas in addition to the existing componentsincreases the initial cost of the fuel cell system. In addition, theknown fuel cell system needs periodical replacement or replenishment ofthe inert gas feeding means such as a nitrogen cylinder, which leads toan increase in the running cost of the fuel cell system.

In this known fuel cell system, just after the start of a powergenerating operation, the fuel gas containing a high concentration ofcarbon monoxide is supplied from the reformer to the fuel cell. Thereason for this is that the operating temperature of the reformer hasnot reached a specified value at the start of a power generatingoperation and therefore a sufficient amount of carbon monoxide cannot beremoved from the fuel gas. If a solid polymer electrolyte fuel cell, forexample, is supplied with the fuel gas containing a high concentrationof carbon monoxide, the catalyst of the fuel electrode of the solidpolymer electrolyte fuel cell will be poisoned by the carbon monoxidesupplied. The poisoning of the catalyst of the fuel electrodesignificantly hinders the progress of the electrochemical reaction inthe fuel cell. Therefore, the known fuel cell system suffers from theproblem that the power generating performance of the fuel celldeteriorates, depending upon the number of stops and starts of powergenerating operation.

As a fuel cell system that can be easily used in the home and electriccars and is unlikely to cause progressive poisoning of the catalyst,there has been proposed a fuel system that is configured to cut off thesupply of the fuel gas to the fuel cell just after the start of a powergenerating operation and to introduce, as a substitution gas, the rawmaterial of the fuel gas into the fuel cell after stopping the powergenerating operation (e.g., Patent Document 1).

The proposed fuel cell system is comprised of: a reformer for generatinga hydrogen-rich fuel gas from a raw material containing as a chiefcomponent an organic compound that contains carbon and hydrogen; a fuelgas feed passage for feeding a fuel gas from the reformer to a fuelcell; an off gas feed passage for feeding the fuel gas, which has beendischarged from the fuel cell without being used in power generation(hereinafter referred to as “off gas”), to a combustion burner of thereformer; and a first bypass route provided between the fuel gas feedpassage and the off gas feed passage, for switching the destination ofthe fuel gas from the fuel cell to the combustion burner of thereformer. This fuel cell system also comprises a raw material feeder forfeeding the raw material of the fuel gas to the reformer and a secondbypass route for feeding the raw material directly from the raw materialfeeder to the fuel cell, so as to bypass the reformer.

In the proposed fuel cell system, just after the start of a powergenerating operation, the fuel gas containing a high concentration ofcarbon monoxide and generated in the reformer is supplied to thecombustion burner of the reformer by way of the first bypass route. Inthe combustion burner, the fuel gas is combusted to heat the reformingcatalyst. After the temperature of the reforming catalyst in thereformer has reached a specified value after the start of a powergenerating operation, the fuel gas generated in the reformer is suppliedto the fuel cell by way of the fuel gas feed passage. Then, the fuel gasis used as a fuel for power generation in the fuel cell. The off gasdischarged from the fuel cell is supplied to the combustion burner ofthe reformer by way of the off gas feed passage. Then, the off gas iscombusted for heating the reforming catalyst in the combustion burner.

In the proposed fuel cell system, after stopping the power generatingoperation, the raw material serving as a substitution gas is introducedfrom the raw material feeder to a fuel gas flow path of the fuel cell byway of the second bypass route. Thereby, the inside of the fuel cell andits surrounding area are sealed up by the raw material instead of theinert gas such as nitrogen gas throughout the period during which thepower generating operation of the fuel cell system is stopped.

According to this fuel cell system, the raw material serving as asubstitution gas is introduced into the fuel cell from theconventionally-used raw material feeder after stopping the powergeneration, which eliminates the need for provision of an inert gasfeeding means such as a nitrogen cylinder within or in the proximity ofthe fuel cell system. The fuel cell system can avoid growing in size andtherefore can be used as a stationary dispersed power generation systemfor household use or an electrical vehicular power plant. In addition,since there is no need to provide an inert gas (nitrogen gas) feedingmeans in addition to existing components, the initial cost of the fuelcell system can be kept low. Furthermore, periodical replacement of aninert gas (nitrogen gas) feeding means is unnecessary, which makes itpossible to keep the running cost of the fuel cell system low.

The material introduced from the raw material feeder into the fuel cellis chemically stable compared to hydrogen contained in the fuel gas.Therefore, even if the raw material, stagnating in the fuel cellthroughout a power generating operation stop period, is mixed with airpenetrating thereinto, no rapid oxidizing reaction will occur.Accordingly, damage to the fuel cell system caused by reaction heatgenerated in the oxidizing reaction can be effectively prevented, byintroducing the raw material into the fuel cell. In this way, the fuelcell system can ensure security in the power generating operation stopperiod.

In the proposed fuel cell system, the fuel gas containing a highconcentration of carbon monoxide is not supplied to the fuel cell justafter the start of a power generating operation. After the temperatureof the reforming catalyst in the reformer has reached the specifiedtemperature and the fuel gas whose carbon monoxide concentration hasbeen sufficiently reduced is generated, the fuel gas is supplied fromthe reformer to the fuel gas. This prevents the poisoning of thecatalyst of the fuel electrode in the solid polymer electrolyte fuelcell. Thus, the factors that interrupt the progress of theelectrochemical reaction in the fuel cell are swept away and thereforethe problem that the power generating performance of the fuel celldeteriorates depending on the number of stops and starts of powergenerating operation can be solved.

Patent Document 1: Japanese Laid-Open Patent Application Publication No.2003-229149

DISCLOSURE OF THE INVENTION Problems that the Invention Intends to Solve

However, the above previous proposal has revealed the following problem.When a supply of the fuel gas from the reformer to the fuel cell startsafter the temperature of the reforming catalyst in the reformer hasreached a specified value, the raw material, which has been introducedfrom the raw material feeder into the fuel cell after stopping the powergenerating operation, is forced out of the fuel cell by the fuel gasbeing fed from the reformer and is supplied to the combustion burner ofthe reformer over a specified period of time. Because of this, oxygenruns out in the combustion burner, causing incomplete combustion andtherefore carbon monoxide is released to the atmosphere during thespecified period.

More concretely, the combustion burner in the reformer basicallycombusts hydrogen contained in the off gas in order to promote the steamreforming reaction. For complete combustion of hydrogen, the amount ofair corresponding to the feed rate of hydrogen is fed from thecombustion fan adjacent to the combustion burner.

When a supply of the fuel gas from the reformer to the fuel cell startsafter the temperature of the reforming catalyst in the reformer hasreached a specified value, the combustion burner is supplied with theraw material (e.g., natural gas) discharged from the fuel cell for aspecified period of time, the raw material containing an organiccompound composed of at least carbon and hydrogen. To completely combustthis raw material, an amount of air more than the amount necessary forcomplete combustion of hydrogen is required. However, the feed rate ofair supplied from the combustion fan to the combustion burner isdetermined to be equal to the feed rate of air necessary for completecombustion of hydrogen as stated earlier. Therefore, oxygen shortageoccurs, accompanied with incomplete combustion of the raw materialduring the specified period of time in the combustion burner. As aresult, the combustion burner discharges carbon monoxide in thespecified period during which the raw material such as natural gas issupplied to the combustion burner.

As has been described above, the above proposed fuel cell system has theproblem that carbon monoxide is discharged from the system to theatmosphere for a specified period of time after starting a supply of thefuel gas from the reformer to the fuel cell at the start of powergenerating operation. Carbon monoxide is known to be significantly toxicto human beings. For instance, carbon monoxide combines with hemoglobincontained in blood thereby generating carbonyl hemoglobin, whichconsiderably disturbs the oxygen transfer function of hemoglobin.Therefore, there exists a concern that human life may be adverselyaffected if a large number of such fuel cell systems come into wide useand discharge large amounts of carbon monoxide to the atmosphere.

The invention has been made taking account of the background describedabove and therefore an object of the invention is to provide anenvironmentally-friendly fuel cell system capable of effectivelysuppressing carbon monoxide emission at the start of a power generatingoperation with a simple structure to mitigate the adverse effect on theecosystem.

Means for Solving the Problems

In accomplishing the above object, there is provided, according to theinvention, a fuel cell system comprising:

a fuel cell configured to generate electric power by use of a fuel gasand an oxidizing gas;

a fuel gas generator configured to generate the fuel gas by reforming araw material through a reforming reaction;

a combustor configured to supply heat to said fuel gas generator topromote the reforming reaction;

a combustion fuel feeder configured to control a feed rate of combustionfuel to said combustor;

an air feeder configured to supply air to said combustor;

a fuel gas passage configured to supply the fuel gas from said fuel gasgenerator to the fuel cell;

an off gas passage configured to supply part of the fuel gas from thefuel cell to said combustor, which part has been left without being usedin the electric power generation;

a bypass passage configured to connect the fuel gas passage and the offgas passage such that the fuel gas generated in said fuel gas generatoris supplied to said combustor, so as to bypass the fuel cell;

a switching valve configured to switch the destination of the fuel gasgenerated in said fuel gas generator between the fuel cell and thebypass passage; and

a controller,

wherein an inside of the fuel cell is filled with the raw materialbefore the controller controls the switching valve so as to switch fromthe bypass passage to the fuel cell to supply the fuel gas generated insaid fuel gas generator to the fuel cell, and

wherein the controller controls the combustion fuel feeder so as toreduce the feed rate of the combustion fuel to the combustor whencontrolling the switching valve so as to switch from the bypass passageto the fuel cell to supply the fuel gas generated in said fuel gasgenerator to the fuel cell.

In the above fuel cell system wherein the inside of the fuel cell isfilled with the raw material before the fuel gas generated in said fuelgas generator is supplied to the fuel cell by switching from the bypasspassage to the fuel cell, the combustion fuel can be supplied to thecombustor at an appropriate feed rate and, in consequence, the emissionof carbon monoxide from the fuel cell system at the start of a powergenerating operation can be effectively suppressed.

The above system may be configured such that the combustion fuel feederis a raw material feeder configured to control a feed rate of the rawmaterial to the fuel gas generator, and the controller controls the rawmaterial feeder so as to reduce the feed rate of the raw material whencontrolling the switching valve so as to switch from the bypass passageto the fuel cell to supply the fuel gas generated in the fuel gasgenerator to the fuel cell.

In accordance with this configuration, the raw material feeder isconfigured to adjust the feed rate of the raw material to the fuel gasgenerator. Therefore, the feed rate of the combustion fuel to thecombustor can be reduced without additionally incorporating a specialdevice into the conventional fuel cell system configuration, bycontrolling the raw material feeder so as to reduce the feed rate of theraw material.

The above system may be configured such that the controller controls thecombustion fuel feeder so as to reduce the feed rate of the combustionfuel to the combustor in accordance with a composition of the rawmaterial that fills the inside of the fuel cell.

In accordance with this configuration, the feed rate of the combustionfuel to the combustor can be reduced according to the composition of theraw material that fills the inside of the fuel cell and therefore alimitation on the types of the raw material that fills the inside of thefuel cell can be eliminated.

The above system may be configured such that the controller controls thecombustion fuel feeder so as to satisfy an air ratio of 1 or more toreduce the feed rate of the combustion fuel to the combustor, whilecontrolling the air feeder so as to maintain a feed rate of the air tothe combustor.

In accordance with the above configuration, when the feed rate of airsupplied from the air feeder to the combustor is maintained, the feedrate of the combustion fuel to the combustor is reduced so as to satisfyan air ratio of 1 or more. Therefore, the emission of carbon monoxidefrom the fuel cell system at the start of a power generating operationcan be suppressed without fail.

The above system may be configured such that the controller controls theswitching valve such that the fuel gas generated in the fuel gasgenerator is supplied to the combustor by way of the bypass passageuntil the fuel gas generator satisfies a specified operating condition,and when the specified operating condition is satisfied, the controllercontrols the switching valve so as to switch the destination of the fuelgas generated in the fuel gas generator from the bypass passage to thefuel cell and controls the combustion fuel feeder so as to reduce thefeed rate of the combustion fuel to the combustor.

In accordance with this configuration, since the fuel gas containing ahigh concentration of carbon monoxide is not supplied to the fuel cellbut supplied to the combustor until the fuel gas generator satisfies aspecified operating condition, poisoning of the catalyst of the fuelelectrode in the fuel cell can be suppressed. In addition, when the fuelgas generator satisfies the specified operating condition, the fuel gasis supplied to the fuel cell while the feed rate of the combustion fuelto the combustor is reduced. Therefore, the emission of carbon monoxidefrom the fuel cell system at the time of feeding the fuel gas from thefuel gas generator to the fuel cell can be effectively suppressedwithout fail.

The above system may be configured such that the controller controls thecombustion fuel feeder to reduce the feed rate of the combustion fuel tothe combustor, before controlling the switching valve so as to shut offthe bypass passage to allow a supply of the fuel gas from the fuel gasgenerator to the fuel cell.

In accordance with this configuration, since the feed rate of thecombustion fuel to the combustor is reduced before a supply of the fuelgas from the fuel gas generator to the fuel cell becomes possible, theemission of carbon monoxide from the fuel cell system can be effectivelysuppressed without fail.

The above system may be configured such that the controller controls thecombustion fuel feeder so as to increase the feed rate of the combustionfuel to the combustor, after an elapse of a specified period of timeafter the controller controls the combustion fuel feeder so as to reducethe feed rate of the combustion fuel to the combustor.

In accordance with this configuration, since the feed rate of thecombustion fuel to the combustor is increased after an elapse of aspecified period of time, changes in the feed rate of the combustionfuel to the combustor can be properly controlled.

The above system may further comprise a CO detector configured to detectcarbon monoxide contained in an exhaust gas discharged from thecombustor and may be configured such that the controller controls thecombustion fuel feeder so as to increase the feed rate of the combustionfuel to the combustor, when an output value of the CO detector drops toa specified value or less or the concentration of carbon monoxideobtained based on the output value of the CO detector drops to aspecified value or less, after the controller controls the combustionfuel feeder so as to reduce the feed rate of the combustion fuel to thecombustor.

In accordance with this configuration, since the system furthercomprises the CO detector configured to detect carbon monoxide containedin the exhaust gas discharged from the combustor, changes in the feedrate of the combustion fuel to the combustor can be properly controlled.

The above system may be configured such that the controller controls thecombustion fuel feeder so as to reduce the feed rate of the combustionfuel to the combustor in a stepwise fashion involving one or more steps,or in a continuous fashion.

In accordance with this configuration, since the feed rate of the fuelcombustion to the combustor can be ideally reduced, the emission ofcarbon monoxide can be effectively suppressed.

In the above system, the raw material may be hydrocarbon gas.

In accordance with this configuration, natural gas, propane gas and thelike, which are commonly used as hydrocarbon gas, can be used as the rawmaterial. This facilitates construction of a preferred fuel cell systemcapable of suppressing the emission of carbon monoxide at the start of apower generating operation.

The above system may further comprise a raw material feeder configuredto supply the raw material, and may be configured such that thecontroller allows the raw material feeder to supply the raw material tothe fuel cell to fill the inside of the fuel cell with the raw materialin shutdown operation or start-up operation.

In accordance with this configuration, since the fuel cell system hasthe raw material feeder capable of supplying the raw material to theinside of the fuel cell, the inside of the fuel cell can be easilyfilled with the raw material during shutdown or start-up of the fuelcell system.

The above system may further comprise a gas concentration detectorprovided in the combustor or the off gas passage, for detecting aspecified gas concentration, and may be configured such that thecontroller controls the combustion fuel feeder in response to an outputsignal from the gas concentration detector to reduce the feed rate ofthe combustion fuel to the combustor, after controlling the switchingvalve so as to switch from the bypass passage to the fuel cell to allowa supply of the fuel gas generated in the fuel gas generator to the fuelcell.

In accordance with this configuration, the combustor or the off gaspassage is provided with a gas concentration detector for detecting aspecified gas concentration, and the combustion fuel feeder iscontrolled in response to an output signal from the gas concentrationdetector to reduce the feed rate of the combustion fuel to thecombustor. This further facilitates more ideal reduction of the feedrate of the combustion fuel to the combustor so that the emission ofcarbon monoxide can be more effectively suppressed.

The fuel cell system may be configured such that the controller controlsthe combustion fuel feeder so as to reduce the feed rate of thecombustion fuel to the combustor, when the gas concentration detectordetects an increase in raw material concentration.

In accordance with this configuration, since the feed rate of thecombustion fuel to the combustor is reduced if the gas concentrationdetector detects an increase in the raw material concentration, theemission of carbon monoxide from the fuel cell system at the start of apower generating operation can be properly suppressed.

The above system may comprise a flame-rod type flame sensor provided inthe combustor as the gas concentration detector and the raw material maybe a gas containing hydrocarbon. And, the above system may be configuredsuch that the controller controls the combustion fuel feeder to reducethe feed rate of the combustion fuel to the combustor, when the flamesensor detects an increase in the raw material concentration after thecontroller controls the switching valve so as to switch from the bypasspassage to the fuel cell to allow a supply of the fuel gas generated inthe fuel gas generator to the fuel cell.

In accordance with this configuration, the combustor has acommonly-used, flame-rod type flame sensor as the gas concentrationdetector. This enables it to suppress the emission of carbon monoxide atthe start of a power generating operation in a conventional fuel cellsystem configuration without additional use of a gas concentrationdetector.

EFFECTS OF THE INVENTION

According to the invention, an environmentally-friendly fuel cell systemcan be provided in which carbon monoxide emission at the start of apower generating operation can be effectively suppressed with a simplestructure to mitigate the adverse effect on the ecosystem.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that schematically shows a configuration of afuel cell system according to a first embodiment of the invention.

FIG. 2 is a flow chart that schematically shows part of the operation ofthe fuel cell system according to the first embodiment of the invention.

FIG. 3 is diagrams that schematically show changes in the feed rate of araw material supplied from a raw material feeder to a reformer, whereinFIG. 3( a) shows a case where the feed rate of the raw material isreduced by one step; FIG. 3( b) shows a case where the feed rate of theraw material is reduced in a stepwise fashion; and FIG. 3( c) shows acase where the feed rate of the raw material is gradually reduced.

FIG. 4 is a block diagram that schematically shows a configuration of afuel cell system according to a second embodiment of the invention.

FIG. 5 is a flow chart that schematically shows part of the operation ofthe fuel cell system according to the second embodiment of theinvention.

FIG. 6 is a block diagram that schematically shows a configuration of afuel cell system according to a third embodiment of the invention.

FIG. 7 is a flow chart that schematically shows part of the operation ofthe fuel cell system according to the third embodiment of the invention.

EXPLANATION OF REFERENCE NUMERALS

-   1: fuel cell-   1 a: fuel gas flow path-   1 b: oxidizing gas flow path-   2: reformer-   2 a: combustion burner-   2 b: combustion fan-   3: blower-   4: heat exchanger-   5: hot water storage tank-   6 a, 6 b: pump-   7 a, 7 b: on-off valve-   8: three-way valve-   9: CO sensor-   10: raw material feeder-   11: flame sensor-   12: flow rate control valve-   100-300: fuel cell system-   101: controller-   R1: first route-   R2: second route-   R3: third route-   R4: fourth route-   R5: fifth route-   A: first fuel gas passage-   B: second fuel gas passage

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the accompanying drawings, the best mode for carryingout the invention will be described in detail.

First Embodiment

First of all, the details of the configuration of a fuel cell systemaccording to a first embodiment of the invention will be described withreference to the drawings.

FIG. 1 is a block diagram that schematically shows the configuration ofthe fuel cell system according to the first embodiment of the invention.In FIG. 1, the solid lines connecting the components of the fuel cellsystem represent the passages where water, fuel gas, oxidizing gas,electric signals and the like flow respectively. The arrows on the solidlines indicate the flowing directions of the water, fuel gas, oxidizinggas etc., respectively when the fuel cell system is in normal operation.It should be noted that FIG. 1 shows only the components necessary fordescribing the invention and other components are omitted from FIG. 1.

As illustrated in FIG. 1, the fuel cell system 100 of this embodimenthas a fuel cell 1 that serves as the main body of the power generatingsection thereof. As the fuel cell 1, a solid polymer electrolyte fuelcell is employed in this embodiment. The fuel cell 1 performs powergeneration to output a predetermined amount of electric power, using ahydrogen-rich fuel gas and an oxidizing gas (air is usually used), thefuel gas being supplied from a fuel gas generator (described later) to afuel gas flow path 1 a of the fuel cell 1 whereas the oxidizing gas issupplied from a blower 3 (described later) to an oxidizing gas flow path1 b of the fuel cell 1. In other words, the fuel cell 1 directlyconverts the chemical energy of the fuel gas and the oxidizing gas intoelectric energy through a specified electrochemical reaction that ispromoted by a specified reaction catalyst. By such energy conversion,the fuel cell 1 supplies electric energy to the loads connected to thefuel cell system 100.

In this embodiment, the oxidizing gas supplied to the oxidizing gas flowpath 1 b of the fuel cell 1 is preliminarily treated so as to be in aspecified humidified condition, with the moisture of the oxidizing gasthat has been used in power generation within the fuel cell 1. If thedegree to which the oxidizing gas has been humidified is insufficient,part of water stored in a water storage tank (not particularly shown inFIG. 1) is made to be evaporated within the fuel cell 1 to therebyadjust the humidity of the oxidizing gas to a proper level. In addition,the fuel gas that is supplied to the fuel gas flow path 1 a of the fuelcell 1 is preliminarily brought into a specified humidified condition inthe above-mentioned fuel gas generator.

During a power generating operation, the fuel cell 1 generates heatthrough the specified electrochemical reaction for the energyconversion. The heat generated by the fuel cell 1 is retrieved, in acontinuous manner, by cooling water supplied to a cooling water flowpath (not particularly shown in FIG. 1) formed in the fuel cell 1. Theheat retrieved by the cooling water is utilized within a heat exchanger4 (described later) for heating water supplied from a hot water storagetank 5 (described later).

The inner configuration of the fuel cell 1 is the same as of commonlyused solid polymer electrolyte fuel cells and therefore a detailedexplanation thereof is omitted herein.

As shown in FIG. 1, the fuel cell system 100 has at least a reformer 2as the fuel gas generator of the invention. The reformer 2 mainlypromotes steam reforming reaction to generate the hydrogen-rich fuelgas, using water and a raw material. The raw material contains at leastan organic compound composed of carbon and hydrogen and is exemplifiedby hydrocarbon-based components such as natural gas (mainly containingmethane) and propane gas; alcohol such as methanol; and naphthacomponents. The feed rate of the raw material to the reformer 2 iscontrolled by a raw material feeder 10 (described later) that serves asone example of the raw material feeder of the invention. Cutoff/start ofa supply of the raw material to the reformer 2 is carried out by anon-off valve 7 a. Although not particularly shown in FIG. 1, the fuelgas generator includes a reformer for promoting the steam reformingreaction and a shift converter and purifier for reducing carbon monoxidecontained in the fuel gas discharged from the reformer.

The reformer has a reforming catalyst (not particularly shown in FIG. 1)for promoting the steam reforming reaction; a combustion burner 2 a forcombusting off gas discharged, for instance, from the fuel cell 1 inorder to heat the reforming catalyst; and a combustion fan 2 b forfeeding air necessary for the combustion of the off gas etc. within thecombustion burner 2 a from the atmosphere. The combustion burner 2 acombusts at least any of combustion fuels which are the off gasdischarged from the fuel cell 1, the fuel gas generated in the fuel gasgenerator and the raw material fed from the raw material feeder 10.Thereby, heat energy for heating the reforming catalyst of the reformeris generated.

The shift converter includes a shift catalyst for reducing the carbonmonoxide concentration of the fuel gas discharged from the reformerthrough the reaction with water. The purifier includes a CO removalcatalyst for further reducing the carbon monoxide concentration of thefuel gas discharged from the shift converter through an oxidationreaction or methanation reaction. The shift converter and the purifierare respectively operated under the temperature conditions respectivelysuited for the chemical reactions proceeding therein in order toeffectively reduce carbon monoxide contained in the fuel gas.

A detailed explanation of other components of the fuel gas generatorthan the above-described reformer, shift converter and purifier isomitted herein because the inner configuration of the fuel gas generatoris the same as of commonly-used reformers.

As shown in FIG. 1, the fuel cell system 100 includes the raw materialfeeder 10. The raw material feeder 10 is a booster pump for raising thepressure of the raw material such as natural gas supplied form naturalgas infrastructure or the like during the power generating operation ofthe fuel cell system 100 and feeds the raw material to the reformer 2through the above-mentioned on-off valve 7 a. Herein, the output of theraw material feeder 10 is controlled by a controller 101 (describedlater) so that the feed rate of the raw material to the reformer 2 canbe properly adjusted according to need.

As shown in FIG. 1, the fuel cell system 100 includes the blower 3. Theblower 3 suctions air from the atmosphere to supply air to the oxidizinggas flow path 1 b of the fuel cell 1 as the oxidizing gas. As the blower3, a sirocco fan or the like is suitably used.

As shown in FIG. 1, the fuel cell system 100 includes a heat exchanger4. The heat exchanger 4 exchanges heat between the temperature-risencooling water which has been discharged from a cooling water flow path(not particularly shown in FIG. 1) of the fuel cell 1 by the operationof a pump 6 a and the water fed from the hot water storage tank 5(described later) by a pump 6 b for hot water supply, or the like. Thecooling water which has been cooled by the heat exchange in the heatexchanger 4 is again fed to the cooling water flow path of the fuel cell1 by the operation of the pump 6 a.

As shown in FIG. 1, the fuel cell system 100 includes the hot waterstorage tank 5. The hot water storage tank 5 stores the water heated bythe heat exchanger 4. Herein, the water stored in the hot water storagetank 5 is circulated through the heat exchanger 4 by the operation ofthe pump 6 b. At that time, in the heat exchanger 4, the water suppliedfrom the hot water storage tank 5 is heated by the heat of thetemperature-risen cooling water which has been discharged from the fuelcell 1 by the operation of the pump 6 a. The water heated by the heatexchanger 4 is stored in the hot water storage tank 5. The heated waterstored in the hot water storage tank 5 is used for hot water supplyaccording to need.

As illustrated in FIG. 1, the fuel cell system 100 is provided with athree-way valve 8 located in the junction between a first route R1 and afourth route R4 through which the fuel gas generated in the fuel gasgenerator is fed to the fuel gas flow path 1 a of the fuel cell 1. Anon-off valve 7 b is provided in an intermediate location of a fifthroute 5 through which the off gas discharged from the fuel gas flow path1 a of the fuel cell 1 is fed to the combustion burner 2 a of thereformer 2. Provided between the three-way valve 8 and the junctionbetween the fifth route R5 and the third route R3 is a second route R2(bypass route) that allows the fuel gas generated in the fuel gasgenerator to be directly fed to the combustion burner 2 a, so as tobypass the fuel cell 1. As illustrated in FIG. 1, the first route R1,the second route R2 and the third route R3 constitute a first fuel gaspassage A. As illustrated in FIG. 1, the first route R1, the fourthroute R4, the fuel gas flow path 1 a, the fifth route R5 and the thirdroute R3 constitute a second fuel gas passage B. That is, the fuel cellsystem 100 of the first embodiment is configured to directly feed thefuel gas discharged from the fuel gas generator to the combustion burner2 a without passing it to the fuel cell 1 according to need, byoperating the on-off valve 7 b and the three-way valve 8. In thisembodiment, the first route R1 and the fourth route R4 constitute a fuelgas passage for feeding the fuel gas generated in the fuel gas generatorto the fuel gas flow path 1 a of the fuel cell 1. The fifth route R5 andthe third route R3 constitute an off gas passage through which the offgas discharged from the fuel gas flow path 1 a of the fuel cell 1 is fedto the combustion burner 2 a of the reformer 2.

The fuel cell system 100 further includes the controller 101. Thecontroller 101 properly controls the operations of the components of thefuel cell system 100. Although not particularly shown in FIG. 1, thiscontroller 101 includes, for example, a memory, a timer, a centralprocessing unit (CPU) and others. Programs for the respective operationsof the components of the fuel cell system 100 are prestored in thememory of the controller 101 and the controller 101 properly controlsthe operation of the fuel cell system 100 according to the programsstored in the memory.

Next, the operation of the fuel cell system 100 according to the firstembodiment of the invention will be described in detail with referenceto the drawings.

In the following description, it is assumed that during the shutdownoperation or start-up operation of the fuel cell system 100, the fuelgas flow path 1 a of the fuel cell system 100 and its surrounding areaare filled with a raw material beforehand, the raw material (naturalgas, which is hydrocarbon gas, is employed in the first embodiment)serving as a substitution gas and containing at least an organiccompound composed of carbon and hydrogen. Filling the fuel cell 1 etc.with the raw material is done by feeding the raw material from the rawmaterial feeder 10 shown in FIG. 1 to the fuel cell 1 etc. In thisembodiment, the term “during the start-up operation” means “the timeperiod after the controller 101 has output a start-up command until acurrent is taken out from the fuel cell 1 by a power generationcontroller (not particularly shown in FIG. 1) of the fuel cell 1”, andthe term “during the shutdown operation” means “the time period afterthe controller 101 has output a shutdown command until the operation ofthe whole fuel cell system 100 completely stops”.

In the fuel cell system 100, the following operation is performedthrough the control by the controller 101.

First, when starting the power generating operation of the fuel cellsystem 100 shown in FIG. 1, the fuel gas generator is put in operationin order to generate the hydrogen-rich fuel gas necessary for the powergenerating operation of the fuel cell 1. Specifically, natural gas thatis the raw material used for generation of hydrogen is supplied to thereformer 2 by the raw material feeder 10 shown in FIG. 1. To generatesteam to be used for promoting the steam reforming reaction, water issupplied to the reformer 2 from an infrastructure such as a water line.To promote the stream reforming reaction in the reformer 2, a reformingcatalyst provided in the reformer 2 is heated by the combustion burner 2a.

For a short period just after the start of the power generatingoperation of the fuel cell system 100, the reforming catalyst of thereformer 2 is heated by the combustion burner 2 a so as to graduallyraise its temperature, but the temperature of the reforming catalyst hasnot reached a specified value yet. Therefore, the steam reformingreaction in the reformer 2 does not suitably proceed and the fuel gascontaining a large amount of carbon monoxide is discharged from the fuelcell gas generator. In view of this, this embodiment is configured asfollows: After the start of the power generating operation of the fuelcell system 100 until the temperature of the reforming catalyst of thereformer 2 reaches the specified temperature, so that generation of thefuel gas of good quality becomes possible (i.e., a specified operatingcondition is satisfied), the controller 101 controls the three-way valve8 to connect the first route R1 and the second route R2 to each otherand brings the on-off valve into a closed state so that the first routeR1, the second route R2 and the third route R3 establish the first fuelgas passage A. The fuel gas containing a high concentration of carbonmonoxide generated in the fuel gas generator is sent to the first fuelgas passage A. Thereby, the fuel gas containing a high concentration ofcarbon monoxide is supplied to the combustion burner 2 a through thefirst fuel gas passage A. Then, the combustion burner 2 a combusts thesupplied fuel gas containing a high concentration of carbon monoxide,thereby heating the reforming catalyst of the reformer 2. In doing so,the temperature of the reforming catalyst rises to the specified value.It should be noted that the fuel gas combusted by the combustion burner2 a is discharged outside the fuel cell system 100 as exhaust gas.

To combust the fuel gas containing a high concentration of carbonmonoxide in the combustion burner 2 a, the combustion fan 2 b suppliesair to the combustion burner 2 a. The feed rate of air supplied from thecombustion fan 2 b to the combustion burner 2 a is properly set inaccordance with the feed rate of the raw material such as natural gassupplied from the raw material feeder 10 to the reformer 2.

More concretely, in theory, the reformer 2 generates hydrogen fromnatural gas through the chemical reaction represented by Formula (1)after the start of the power generating operation of the fuel cellsystem 100. Herein, where the feed rate of the natural gas supplied tothe reformer 2 by the raw material feeder 10 is designated by Q (L/min.)for convenience sake, the emission of hydrogen from the fuel gasgenerator is 4Q (L/min.) according to the chemical reaction of Formula(1). In this embodiment, in order to perfectly combust hydrogen fed fromthe fuel cell generator to the combustion burner 2 a at the rate of 4Q(L/min.) through the first fuel gas passage A, oxygen is supplied fromthe combustion fan 2 b to the combustion burner 2 a at a rate of 2Q(L/min.), thereby promoting the combustion reaction represented byFormula (2). At that time, the controller 101 controls the rotationalspeed of the combustion fan 2 b so as to make the feed rate of oxygen tothe combustion burner 2 a equal to 2Q (L/min.).

CH₄+2H₂O→CO₂+4H₂  (1)

4H₂+2O₂→4H₂O  (2)

That is, in this embodiment, the feed rate of air supplied from thecombustion fan 2 b to the combustion burner 2 a is set based on theamount of hydrogen theoretically generated by the reformer 2, i.e., thefeed rate of the natural gas supplied from the raw material feeder 10 tothe reformer 2. Thereby, the fuel gas containing a high concentration ofcarbon monoxide can be combusted by the combustion burner 2 a. And, thereforming catalyst of the reformer 2 is heated by the heat energygenerated in the combustion burner 2 a.

The subsequent operation will be described in detail with reference toFIG. 2 to more concretely explain the invention.

FIG. 2 is a flow chart that schematically shows part of the operation ofthe fuel cell system according to the first embodiment of the invention.

As shown in FIG. 2, after the reforming catalyst of the reformer 2 hasrisen in temperature owing to the heat generated from the combustion ofthe fuel gas containing a high concentration of carbon monoxide in thecombustion burner 2 a, the controller 101 determines whether or not thetemperature of the reforming catalyst has reached the specifiedtemperature suited for the steam reforming reaction (Step Si). Herein,the temperature of the reforming catalyst is detected by, for example, atemperature sensor embedded in the reforming catalyst. The output signalof the temperature sensor is input to the controller 101. Then, thecontroller 101 analyses the output signal, thereby recognizing thetemperature of the reforming catalyst. If it is determined that thetemperature of the reforming catalyst has not reached the specifiedtemperature (“NO” at Step S1), heating of the reforming catalyst by thecombustion burner 2 a will be continued until it is determined that thetemperature of the reforming catalyst has reached the specifiedtemperature.

If the controller 101 determines at Step S1 that the temperature of thereforming catalyst has reached the specified temperature (“YES” at StepS1), the controller 101 then controls the raw material feeder 10 toreduce the feed rate of the raw material to the reformer 2 whilemaintaining the volume of air sent from the combustion fan 2 b (StepS2).

More concretely, the feed rate of the natural gas, which is supplied tothe combustion burner 2 a after being discharged (forced out) from thefuel gas flow path 1 a etc. of the fuel cell 1 subsequently to Step S3(described later), is approximately equal to the feed rate of the fuelgas supplied from the fuel gas generator to the fuel gas flow path 1 a.For example, according to Formula (1), when the feed rate of the naturalgas to the reformer 2 is Q (L/min.), the fuel gas generator dischargescarbon dioxide at a rate of Q (L/min.) and hydrogen at a rate of 4Q(L/min.). Therefore, natural gas is supplied from the fuel gas flow path1 a etc. of the fuel cell 1 to the combustion burner 2 a at a rate of 5Q(L/min.).

To completely combust the natural gas supplied at a rate of 5Q (L/min.)thereby converting the natural gas into carbon dioxide and water asrepresented by Formula (3), it is necessary to supply oxygen to thecombustion burner 2 a at a rate of 10Q (L/min.). However, at the startof the power generating operation of the fuel cell system 100, the feedrate of oxygen supplied to the combustion burner 2 a is 2Q (L/min.)depending on the feed rate of the natural gas supplied to the reformer2, as discussed earlier. Therefore, incomplete combustion of thesupplied natural gas proceeds in the combustion burner 2 a, which causescarbon monoxide emission from the fuel cell system 100.

5CH₄+10O₂→5CO₂+10H₂O  (3)

In this embodiment, for complete combustion of the natural gasdischarged from the fuel gas flow path 1 a etc. of the fuel cell 1 andsupplied to the combustion burner 2 a, the feed rate of the raw materialto the reformer 2 is properly reduced by the raw material feeder 10 atstep S2 before the establishment of the second fuel gas passage B bycontrolling the on-off valve 7 b and the three-way valve 8 at step S3.In this way, the amount of natural gas, which is forced out of the fuelgas flow path 1 a etc. of the fuel cell 1 and supplied to the combustionburner 2 a in the case where the second fuel gas passage B isestablished, is properly reduced to the value corresponding to theamount of oxygen supplied at a rate of 2Q (L/min.).

In this embodiment, the feed rate of the raw material supplied from theraw material generator 10 to the reformer 2 is reduced to about ⅕ (about2/10) according to Formula (3). Therefore, the feed rate of the naturalgas supplied to the combustion burner 2 a becomes Q (L/min.). Therefore,even if the feed rate of oxygen supplied from the combustion fan 2 b tothe combustion burner 2 a is kept to 2Q (L/min.), the natural gassupplied at a rate of Q (L/min.) can be substantially completelycombusted in the combustion burner 2 a so that the carbon monoxideemission to the outside of the fuel cell system 100 is suppressed.

In other words, in this embodiment, the controller 101 properly controlsthe operation of the raw material feeder 10 so as to reduce the feedrate of natural gas to the combustion burner 2 a while keeping the feedrate of air supplied from the combustion fan 2 b to the combustionburner 2 a, so that an air ratio of 1 or more is satisfied. “The airratio” stated herein means the ratio of the actual amount of air to thetheoretical amount of air necessary for complete combustion of thecombustion fuel.

Herein, the feed rate of the raw material supplied to the reformer 2 maybe reduced by the raw material feeder 10 in any patterns.

FIG. 3 is diagrams each schematically showing a reduction pattern forthe feed rate of the raw material supplied from the raw material feeder10 to the reformer 2. In FIGS. 3( a) to 3(c), the feed rate of the rawmaterial to the reformer 2 which feed rate is controlled by the rawmaterial feeder 10 is plotted on the ordinate whereas the time elapsedis plotted on the abscissa.

As shown in FIG. 3, at Step S2 shown in FIG. 2, the feed rate of the rawmaterial supplied from the raw material feeder 10 to the reformer 2 maybe reduced by one step as indicated by the curve a shown in FIG. 3( a)or may be reduced in a stepwise fashion as indicated by the curve bshown in FIG. 3( b). Alternatively, it may be gradually reduced asindicated by the curve c shown in FIG. 3( c). With any of the reductionpatterns of FIGS. 3( a) to 3(c), incomplete combustion of natural gas inthe combustion burner 2 a can be effectively suppressed.

After the feed rate of the raw material supplied to the reformer 2 isreduced by the raw material feeder 10 at Step S2, the controller 101controls the three-way valve 8 and the on-off valve 7 b to establish thesecond fuel gas passage B by the first route R1, the fourth route R4,the fuel gas flow path 1 a, the fifth route R5 and the third route R3(Step S3).

By this time, the temperature of the reforming catalyst in the reformer2 has reached a specified value which enables adequate progression ofthe steam reforming reaction, so that the fuel gas generator dischargesthe fuel gas containing a sufficiently reduced amount of carbonmonoxide. Then, the fuel gas generated in the fuel gas generator andcontaining a sufficiently reduced amount of carbon monoxide is suppliedto the fuel gas flow path 1 a etc. of the fuel cell 1 by way of thefirst route R1 and the fourth route R4. Owing to the fuel gas suppliedfrom the fuel gas generator to the fuel gas flow path 1 a etc. of thefuel cell 1, the natural gas, which has been introduced into the fuelgas flow path 1 a of the fuel cell 1 and its surrounding area as asubstitution gas, is forced out. This natural gas is supplied to thecombustion burner 2 a by way of the fifth route R5 and the third routeR3.

In the combustion burner 2 a, the natural gas forced out of the fuel gasflow path 1 a etc. of the fuel cell 1 is combusted with air fed from thecombustion fan 2 b. At that time, the amount of oxygen required forcomplete combustion of the raw material (natural gas) is supplied fromthe combustion fan 2 b by reducing the feed rate of the raw material tothe reformer 2 as described earlier, and therefore the natural gas iscompletely combusted in the combustion burner 2 a. In this way, carbonmonoxide emission to the outside of the fuel cell system 100 issuppressed.

After the feed rate of the raw material to the reformer 2 is reduced bythe raw material feeder 10 to reduce the feed rate of the natural gas tothe combustion burner 21, the whole amount of natural gas is dischargedfrom the fuel gas flow path 1 a etc. of the fuel cell 1. If the timer ofthe controller 101 determines that a specified time required forcombusting the whole amount of natural gas in the combustion burner 2 ahas elapsed (“YES” at Step S4), the feed rate of the raw material to thereformer 2 is increased by the raw material feeder 10 (Step S5).

More specifically, the controller 101 controls the operation of the flowrate control section of the raw material feeder 10 in order that thefeed rate of the raw material supplied from the raw material feeder 10to the reformer 2 is changed from ⅕Q (L/min.) to 2Q (L/min.), therebyrestoring the raw material feed rate before reduction. At Step S5 andlater steps, the combustion burner 2 a combusts the off gas dischargedfrom the fuel gas flow path 1 a of the fuel cell 1, so that thereforming catalyst of the reformer 2 is kept at the specifiedtemperature that enables progression of the steam reforming reaction.

“The specified time” determined to be “YES” at step S4 is defined as T(min.) that is calculated from Formula (4) where the sum of thevolumetric capacities of the fuel gas flow path 1 a of the fuel cell 1,the fifth route R5 and the third route R3 is denoted by V(L) and thefeed rate of the natural gas to the combustion burner 2 a is denoted byX(L/min.).

T≧V/X  (4)

At Step S3 and later steps, the fuel gas is supplied from the fuel gasgenerator to the fuel cell 1 so that the fuel cell 1 starts a powergenerating operation in the following way.

Specifically, after the fuel gas whose carbon monoxide concentration hasbeen sufficiently reduced is supplied from the fuel gas generator to thefuel gas flow path 1 a of the fuel cell 1 and air is supplied from theblower 3 to the oxidizing gas flow path 1 b of the fuel cell 1, powergeneration is performed in the fuel cell 1, using the fuel gas and airsupplied to the anode side and cathode side thereof to output aspecified amount of electric power. The off gas, which has not been usedfor power generation, is discharged from the fuel gas flow path 1 a ofthe fuel cell 1 and then supplied to the combustion burner 2 a by way ofthe fifth route R5 and the third route R3. Then, the off gas iscombusted in the combustion burner 2 a, for promoting the steamreforming reaction. The air discharged from the oxidizing gas flow path1 b of the fuel cell 1 is discharged outside the fuel cell system 100.

During the power generating operation, the fuel cell 1 generates heatthrough the electrochemical reaction for power generation. The heatgenerated in the fuel cell 1 is successively retrieved by cooling waterthat is circulated, by the pump 6 a, in the cooling water flow path (notparticularly shown in FIG. 1) formed in the fuel cell 1. The heat,retrieved by the cooling water circulated by the pump 6 a, is utilizedwithin the heat exchanger 4 to heat the water circulated by the pump 6 bfrom the hot water storage tank 5.

Although the first embodiment has been described with a case wherenatural gas is used as the raw material for generating the fuel gas andthe fuel gas flow path 1 a of the fuel cell 1 and its surrounding areaare preliminarily filled with natural gas serving as a substitution gas,the invention is not necessarily limited to this. Alternatively, propanegas may be used as the raw material for generating the fuel gas and thefuel gas flow path 1 a of the fuel cell 1 may be preliminarily filledwith propane gas serving as a substitution gas.

In this case, in the reformer 2, hydrogen is generated from propane gasand water through the chemical reaction represented by Formula (5). Ifthe feed rate of propane gas supplied from the raw material feeder 10 tothe reformer 2 is Q (L/min.), the emission of hydrogen from the fuel gasgenerator is 10Q (L/min.) according to the chemical reaction representedby Formula (5). For complete combustion of hydrogen supplied to thecombustion burner 2 a at a rate of 10Q (L/min.), oxygen may be suppliedat a rate of 5Q (L/min.) based on Formula (6). In doing so, thecontroller 101 controls the rotational speed of the combustion fan 2 bsuch that the feed rate of oxygen to the combustion burner 2 a becomes5Q (L/min.).

C₃H₈+6H₂O→3CO₂+10H₂  (5)

10H₂+5O₂→10H₂O  (6)

In the above case, if Step S3 shown in FIG. 2 is executed and the secondfuel gas passage B is established by controlling the on-off valve 7 band the three-way valve 8, it becomes necessary to supply oxygen at arate of 65Q (L/min.) based on Formula (7), for complete combustion ofthe propane gas supplied to the combustion burner 2 a at a rate of 13Q(L/min.). To this end, the feed rate of the propane gas to the reformer2 is reduced by the raw material feeder 10 at Step S2 prior to theestablishment of the second fuel gas passage B by controlling the on-offvalve 7 b and the three-way valve 8 at Step S3 of FIG. 2, whereby thefeed rate of the propane gas to the combustion burner 2 a is reduced.More concretely, the feed rate of the propane gas supplied from the rawmaterial feeder 10 to the reformer 2 is reduced to about 1/13 (about5/65), thereby reducing the feed rate of the propane gas to thecombustion burner 2 a to about 1/13.

13C₃H₈+65O₂→39CO₂+42H₂O  (7)

As should appreciated, a technical feature of the invention is that thefeed rate of the raw material to the reformer 2 is reduced by the rawmaterial feeder 10 for a specified period of time depending on the typeof substitution gas introduced into the fuel cell 1.

Although the first embodiment has been described with a case where thefeed rate of the raw material to the reformer 2 is reduced by the rawmaterial feeder 10 before the establishment of the second fuel gaspassage B, the invention is not limited to this. Alternatively, the feedrate of the raw material to the reformer 2 could be reduced by the rawmaterial feeder 10 after or at the same time with the establishment ofthe second fuel gas passage B. With this modification, the same effectas of the first embodiment can be obtained. In the case where the feedrate of the raw material to the reformer 2 is reduced after theestablishment of the second fuel gas passage B, the reduction should bedone by the raw material feeder 10 before the natural gas forced out ofthe fuel cell 1 etc. is supplied to the combustion burner 2 a by way ofthe fifth route R5 and the third route R3.

Although the first embodiment has been described with a case where thetemperature of the reforming catalyst is detected at Step S1 of FIG. 2,the invention is not necessarily limited to this but is equallyapplicable to a case where the operating temperature of at least any oneof the components, i.e., the reformer, shift converter and purifier ofthe fuel gas generator is detected. With this modification, the sameeffect as of the first embodiment can be obtained.

Although the fuel cell system 100 has a solid polymer electrolyte fuelcell as the fuel cell 1 in the first embodiment, the fuel cell 1 is notlimited to this but could be a phosphoric-acid fuel cell, an alkalinefuel cell, etc. With this modification, the same effect as of the firstembodiment can be obtained.

Second Embodiment

FIG. 4 is a block diagram that schematically shows a configuration of afuel cell system according to a second embodiment of the invention. InFIG. 4, the solid lines connecting the components of the fuel cellsystem represent the passages where water, fuel gas, oxidizing gas andthe like flow respectively. The arrows on the solid lines indicate theflowing directions of the water, fuel gas, oxidizing gas etc.respectively when the fuel cell system is in normal operation. It shouldbe noted that FIG. 4 shows only the components necessary for describingthe invention and other components are omitted from FIG. 4. In FIG. 4,the same components as of the fuel cell system 100 of the firstembodiment are identified by the same reference numerals as in the firstembodiment.

As illustrated in FIG. 4, a fuel cell system 200 according to thisembodiment has substantially the same configuration as of the fuel cellsystem 100 of the first embodiment. The fuel cell system 200 of thisembodiment differs from the fuel cell system 100 of the first embodimentin that the fuel cell system 200 has a CO sensor 9. Except this, theconfiguration of the fuel cell system 200 is the same as of the fuelcell system 100 of the first embodiment.

As stated above, the fuel cell system 200 of the second embodiment has aCO sensor 9. The CO sensor 9 outputs, as a change in electric signal, achange in the carbon monoxide concentration of the exhaust combustiongas released from the combustion burner 2 a. The controller 101 analyzesthe electric signal output from the CO sensor 9 to detect, for instance,a change in the carbon monoxide concentration of the exhaust combustiongas. In this embodiment, the determination on “a specified period oftime” at Step S4 of FIG. 2 is replaced by the process in which if thecontroller 101 determines that the carbon monoxide concentration of theexhaust combustion gas released from the combustion burner 2 a hasbecome equal to or lower than “a specified threshold concentration”, theoutput of the raw material feeder 10 is controlled so as to increase thefeed rate of the raw material to the reformer 2.

More specifically, when the natural gas that has filled the fuel gasflow path 1 a of the fuel cell 1 and its surrounding area is combustedin the combustion burner 2 a, carbon dioxide and water are generated aschief products as indicated by Formula (3) whereas a slight amount ofcarbon monoxide is generated in case of incomplete combustion. In thisembodiment, if the carbon monoxide concentration of the exhaustcombustion gas detected by the CO sensor 9 drops from, for example, 10ppm to a value equal to or lower than a specified thresholdconcentration (3 ppm), the feed rate of the raw material to the reformer2 is increased by the raw material feeder 10.

FIG. 5 is a flow chart that schematically shows part of the operation ofthe fuel cell system according to the second embodiment of theinvention.

As shown in FIG. 5, in the second embodiment, if it is determined thatthe temperature of the reforming catalyst of the reformer 2 has reacheda specified value (“YES” at Step S1), the controller 101 controls theraw material feeder 10 so as to reduce the feed rate of the raw materialto the reformer 2, similarly to the first embodiment (Step S2).Thereafter, the controller 101 controls the on-off valve 7 b and thethree-way valve 8 to establish the second fuel gas passage B by thefirst route R1, the fourth route R4, the fuel gas flow path 1 a, thefifth route R5 and the third route R3 (Step S3). In this embodiment, asshown in FIG. 5, if it is determined that the carbon monoxideconcentration of the exhaust combustion gas released from the combustionburner 2 a has become equal to or lower than “the specified thresholdconcentration” (“YES” at Step S4), the controller 101 controls the rawmaterial feeder 10 so as to increase the feed rate of the raw materialto the reformer 2 (Step S5).

With the above configuration, the feed rate of the raw material to thereformer 2 can be increased by the raw material feeder 10 after thecarbon monoxide concentration of the exhaust combustion gas has becomeequal to or lower than the specified threshold concentration and thecombustion of the natural gas serving as a substitution gas hasdetermined to be completed.

Although the second embodiment has been described with a case where thefeed rate of the raw material to the reformer 2 is increased by the rawmaterial feeder 10 based on the carbon monoxide concentration of theexhaust combustion gas released from the combustion burner 2 a, theinvention is not limited to this. For example, the feed rate of the rawmaterial to the reformer 2 could be increased by the raw material feeder10 based on the output value of an electric signal released from the COsensor 9 instead of the carbon monoxide concentration of the exhaustcombustion gas released from the combustion burner 2 a.

More concretely, in the fuel cell system 200 of the second embodiment,when the CO sensor 9 outputs a change in the carbon monoxideconcentration of the exhaust combustion gas released from the combustionburner 2 a to the controller 101 as a change in electric signal, thecontroller 101 detects the output value (e.g., voltage value) of theelectric signal released from the CO sensor 9. Then, if the controller101 determines that the output value from the CO sensor 9 indicative ofthe carbon monoxide concentration of the exhaust combustion gasdischarged from the combustion burner 2 a has become equal to or lowerthan “a specified output value”, instead of the determination based onthe “specified period of time” at step S4 in FIG. 2 or the determinationbased on the “specified threshold concentration” at step S4 in FIG. 5,the feed rate of the raw material to the reformer 2 is increased by theraw material feeder 10. This configuration enables it to eliminate theneed for calculation of the carbon monoxide concentration of the exhaustcombustion gas discharged from the combustion burner 2 a by thecontroller 101, and therefore the programs prestored in the memory ofthe controller 101 can be simplified.

The second embodiment does not differ from the first embodiment exceptthe above point.

Third Embodiment

In the first and second embodiments, the one-step reduction pattern, thestepwise reduction pattern and the gradual reduction pattern are shownas the reduction patterns for the feed rate of the raw material suppliedfrom the raw material feeder 10 to the reformer 2. For ideal combustionreaction in the combustion burner 2 a, a more desirable pattern is suchthat the reduction of the feed rate of the raw material supplied fromthe raw material feeder 10 to the reformer 2 is carried out inaccordance with the composition of the combustion fuel supplied to thecombustion burner 2 a to prevent occurrence of accidental fire orincomplete combustion in the combustion burner 2 a.

Instead of the patterns configured to simply reduce the feed rate of theraw material to the reformer 2 by the raw material feeder 10, thefollowing pattern is preferably employed: When the three-way valve 8,which is the switching means of the invention, changes the destinationof the fuel gas generated in the reformer 2 from the second route R2 tothe fuel cell 1, the composition (that is, in fact, the compositionratio of hydrogen and natural gas) of the combustion fuel supplied tothe combustion burner 2 a is successively detected by a specifieddetecting means, and in accordance with the result of the detection, thefeed rate of the raw material to the reformer 2 is properly reduced bythe raw material feeder 10. If either the hydrogen concentration or theraw material concentration in the off gas routes R3, R5 or thecombustion burner 2 a of the invention is detected, the otherconcentration is predictable. Therefore, in this embodiment, thecombustion burner 2 a is provided with a flame-rod type flame sensor 11as the raw material concentration detector of the invention, and if theraw material concentration detected by the flame sensor 11 increases,the feed rate of the combustion fuel (raw material) to the combustionburner 2 a is reduced. The details will be described below.

FIG. 6 is a block diagram that schematically shows a configuration of afuel cell system according to a third embodiment of the invention. InFIG. 6, the solid lines connecting the components of the fuel cellsystem represent the passages where water, fuel gas, oxidizing gas andthe like flow respectively. The arrows on the solid lines indicate theflowing directions of the water, fuel gas, oxidizing gas etc.respectively when the fuel cell system is in normal operation. It shouldbe noted that FIG. 6 shows only the components necessary for describingthe invention and other components are omitted from FIG. 6. In FIG. 6,the same components as of the fuel cell system 100 of the firstembodiment are identified by the same reference numerals as in the firstembodiment.

As illustrated in FIG. 6, a fuel cell system 300 according to thisembodiment has substantially the same configuration as of the fuel cellsystem 100 of the first embodiment.

The configuration of the fuel cell system 300 of this embodiment differsfrom that of the fuel cell system 100 of the first embodiment in thatthe fuel cell system 300 has the flame-rod type flame sensor 11 providedwithin the combustion burner 2 a as the raw material concentrationdetector of the invention. In addition, the configuration of the fuelcell system 300 differs from that of the fuel cell system 100 of thefirst embodiment in that the fuel cell system 300 has a flow ratecontrol valve 12 and the raw material feeder 10 is able to feed the rawmaterial to the combustion burner 2 a through the flow rate controlvalve 12. Except these points, the configuration of the fuel cell system300 is the same as of the fuel cell system 100 of the first embodiment.

As just stated, the fuel cell system 300 of the third embodimentincludes the flame sensor 11. The flame sensor 11 detects an ion currentgenerated when hydrocarbon contained in the raw material is combusted inthe combustion burner 2 a and outputs an electric signal indicative ofthe magnitude of the ion current to the controller 101. The controller101 analyses the electric signal output from the flame sensor 11,thereby detecting the raw material concentration of the gas supplied tothe combustion burner 2 a and, therefore, the composition of thecombustion fuel. In this embodiment, the raw material feeder 10 properlyreduces the feed rate of the raw material to the reformer 2 according tothe detected composition of the combustion fuel.

FIG. 7 is a flow chart that schematically shows part of the operation ofthe fuel cell system according to the third embodiment of the invention.

As shown in FIG. 7, in the third embodiment, if it is determined thatthe temperature of the reforming catalyst of the reformer 2 has reacheda specified value (“YES” at Step S1), the controller 101 controls theon-off valve 7 b and the three-way valve 8 to establish the second fuelgas passage B by the first route R1, the fourth route R4, the fuel gasflow path 1 a, the fifth route R5 and the third route R3, similarly tothe first embodiment (Step S2).

Then, the controller 101 causes the flame sensor 11 to continuouslydetect the hydrocarbon concentration of the combustion fuel supplied tothe combustion burner 2 a, estimates a change (an increase in the rawmaterial concentration) in the composition of the combustion fuel fromthe increase in the detected hydrocarbon concentration, and controls theraw material feeder 10 so as to gradually reduce the feed rate of theraw material to the reformer 2 according to the change in thecomposition of the combustion fuel. Herein, a data table for the properraw material feed rate corresponding to the composition of thecombustion fuel is prestored in the memory (not shown in FIG. 6) of thecontroller 101. Based on the data of this prestored data table, thecontroller 101 controls the raw material feeder 10 to properly reducethe feed rate of the raw material to the reformer 2 (Step S3).

Thereafter, if the whole amount of the raw material (natural gas) isdischarged from the fuel gas flow path 1 a of the fuel cell 1 and thetimer of the controller 101 determines that the time required forcombusting the whole amount of the raw material (natural gas) in thecombustion burner 2 a has elapsed (“YES” at Step S4), the controller 101then controls the raw material feeder 10 so as to increase the feed rateof the raw material to the reformer 2 (Step S5).

The above configuration enables it to effectively prevent accidentalfire and incomplete combustion in the combustion burner 2 a, because thefeed rate of the raw material to the reformer 2 is properly reduced bythe raw material feeder 10 in accordance with a change (an increase inthe raw material concentration) in the composition of the combustionfuel supplied to the combustion burner 2 a which change has beendetected by the frame sensor 11 that serves as the raw materialconcentration detector of the invention. Thereby, ideal progression ofthe combustion reaction in the combustion burner 2 a becomes possible inthe transition period in which feeding of the fuel gas to the fuel cell1 starts, the fuel gas being generated in the reformer 2 that serves asthe fuel gas generator.

In the fuel cell system 300 of the third embodiment described above, thecombustion burner 2 a has the flame-rod type flame sensor 11 to detectthe raw material concentration of the combustion fuel and check a changein the composition of the combustion fuel. This could be replaced withthe following alternative configuration. The off-gas passage (the thirdroute R3 or the fifth route R5) of the invention is provided with ahydrogen concentration detector serving as the gas concentrationdetector of the invention, and a change in the composition of thecombustion fuel (an increase in the raw material concentration) isestimated based on a decrease in the hydrogen concentration detected bythe hydrogen concentration detector. According to the estimated changein the composition of the combustion fuel, the output of the rawmaterial feeder 10 is controlled so as to properly reduce the feed rateof the raw material to the combustion burner 2 a.

In the fuel cell system 300 of the third embodiment described above, thecomposition of the combustion fuel is estimated based on the rawmaterial concentration detected by the flame sensor 11 that serves asthe raw material concentration detector of the invention, and the feedrate of the raw material is reduced to an appropriate feed rate byreferring to the table data relating to the appropriate material feedrate according to the composition of the combustion fuel stored in thememory, based on the estimated composition of the combustion fuel. Thismay be replaced with the following alternative configuration. Therelationship between the raw material concentration detected by the rawmaterial concentration detector and the appropriate feed rate of the rawmaterial is prestored in the form of table data in the memory, and theoptimum raw material feed rate is determined directly from the rawmaterial concentration detected by the raw material detector withoutestimating the composition of the combustion fuel. This configuration isapplicable to the case where a change in the composition of thecombustion fuel is estimated, using the hydrogen concentration detectorinstead of the above-described raw material concentration detector.

A modification of this embodiment will be described.

In this modification, during a start-up operation of the fuel cellsystem 300, not only the fuel gas generated in the fuel gas generator issupplied but also natural gas is supplied as assist gas from the rawmaterial feeder 10 to the combustion burner 2 a through the flow ratecontrol valve 12.

Instead of the above configuration in which the feed rate of the rawmaterial to the reformer 2 is properly reduced by the raw materialfeeder 10 according to a change in the composition of the combustionfuel supplied to the combustion burner 2 a (an increase in the rawmaterial concentration), an alternative configuration may be employedaccording to which the feed rate of the raw material supplied from theraw material feeder 10 to the combustion burner 2 a is properly reducedby reducing the open degree of the flow rate control valve 12 with thecontroller 101 in accordance with a change in the composition of thecombustion fuel supplied to the combustion burner 2 a (an increase inthe raw material concentration). In this case, at Step S3 shown in FIG.7, the process described by “the feed rate of the raw material to thereformer 2 is gradually reduced by the raw material feeder 10” isreplaced by the process “the feed rate of the raw material to thecombustion burner 2 a is gradually reduced by the flow rate controlvalve 12”. In this modification, the flow rate control valve 12 servesas the combustion fuel feeder of the invention.

This modification may be applied not only to this embodiment but also tothe first and second embodiments.

As described above, according to the invention, when combusting the rawmaterial (natural gas) that serves as a substitution gas in thecombustion burner 2 a, the feed rate of the natural gas to thecombustion burner 2 a is reduced by reducing the feed rate of the rawmaterial to the reformer 2 by the raw material feeder 10, so that thegeneration of carbon monoxide during the combustion of the natural gascan be suppressed. This makes it possible to provide an environmentallyfriendly fuel cell system that effectively suppresses carbon monoxideemission at the start of a power generating operation with a simpleconfiguration to mitigate the adverse effect upon the ecosystem.

As the criterion for the determination on whether or not the feed rateof the raw material to the reformer 2 should be increased by the rawmaterial feeder 10, “a specified period of time” is used in the firstand third embodiments and “a specified threshold concentration” or “aspecified output value” is used in the second embodiment. However, thesecriteria need not to be used individually but may be used incombination.

That is, if the controller 101 recognizes an elapse of the specifiedperiod of time at Step S4 and the carbon monoxide concentration detectedby the CO sensor 9 has reached a value equal to or lower than thespecified threshold value (or the output value of the CO sensor 9 hasreached a value equal to or lower than the specified output value) atFIGS. 2 and 7, the process may proceed to Step S5 shown in FIGS. 2, 7.This configuration achieves the same effect as of the first to thirdembodiments.

INDUSTRIAL APPLICABILITY

The fuel cell systems described according to the embodiments of theinvention are industrially applicable as an environmentally friendlyfuel cell system capable of effectively suppressing carbon monoxideemission at the start of a power generating operation with a simplestructure to mitigate the adverse effect upon the ecosystem.

1. A fuel cell system comprising: a fuel cell configured to generate electric power by use of a fuel gas and an oxidizing gas; a fuel gas generator configured to generate the fuel gas by reforming a raw material through a reforming reaction; a combustor configured to supply heat to said fuel gas generator to promote the reforming reaction; a combustion fuel feeder configured to control a feed rate of combustion fuel to said combustor; an air feeder configured to supply air to said combustor; a fuel gas passage configured to supply the fuel gas from said fuel gas generator to said fuel cell; an off gas passage configured to supply part of the fuel gas from said fuel cell to said combustor, which part has been left without being used in the electric power generation; a bypass passage configured to connect said fuel gas passage and said off gas passage such that the fuel gas generated by said fuel gas generator is supplied to said combustor, so as to bypass said fuel cell; a switching valve configured to switch a destination of the fuel gas generated in said fuel gas generator between said fuel cell and said bypass passage; and a controller, wherein an inside of said fuel cell is filled with the raw material before said controller controls said switching valve so as to switch from said bypass passage to said fuel cell to supply the fuel gas generated in said fuel gas generator to said fuel cell, and wherein said controller controls said combustion fuel feeder so as to reduce the feed rate of the combustion fuel to said combustor when controlling said switching valve so as to switch from said bypass passage to said fuel cell to supply the fuel gas generated in said fuel gas generator to said fuel cell.
 2. The fuel cell system according to claim 1, wherein said combustion fuel feeder is a raw material feeder configured to control a feed rate of the raw material to said fuel gas generator; and wherein said controller controls said raw material feeder so as to reduce the feed rate of the raw material when controlling said switching valve so as to switch from said bypass passage to said fuel cell to supply the fuel gas generated in said fuel gas generator to said fuel cell.
 3. The fuel cell system according to claim 1, wherein said controller controls said combustion fuel feeder so as to reduce the feed rate of the combustion fuel to said combustor in accordance with a composition of the raw material that fills the inside of said fuel cell.
 4. The fuel cell system according to claim 1, wherein said controller controls said combustion fuel feeder so as to satisfy an air ratio of 1 or more to reduce the feed rate of the combustion fuel to said combustor, while controlling said air feeder so as to maintain a feed rate of the air to said combustor.
 5. The fuel cell system according to claim 1, wherein said controller controls said switching valve such that the fuel gas generated in said fuel gas generator is supplied to said combustor by way of said bypass passage until said fuel gas generator satisfies a specified operating condition; and wherein when the specified operating condition is satisfied, said controller controls said switching valve so as to switch the destination of the fuel gas generated in said fuel gas generator from said bypass passage to said fuel cell and controls said combustion fuel feeder so as to reduce the feed rate of the combustion fuel to said combustor.
 6. The fuel cell system according to claim 1, wherein said controller controls said combustion fuel feeder to reduce the feed rate of the combustion fuel to said combustor, before controlling said switching valve so as to shut off said bypass passage to allow a supply of the fuel gas from said fuel gas generator to said fuel cell.
 7. The fuel cell system according to claim 1, wherein said controller controls said combustion fuel feeder so as to increase the feed rate of the combustion fuel to said combustor, after an elapse of a specified period of time after said controller controls said combustion fuel feeder so as to reduce the feed rate of the combustion fuel to said combustor.
 8. The fuel cell system according to claim 1, further comprising a CO detector configured to detect carbon monoxide contained in an exhaust gas discharged from said combustor, wherein said controller controls said combustion fuel feeder so as to increase the feed rate of the combustion fuel to said combustor, when an output value of said CO detector drops to a specified value or less or a concentration of carbon monoxide obtained based on the output value of said CO detector drops to a specified value or less, after said controller controls said combustion fuel feeder so as to reduce the feed rate of the combustion fuel to said combustor.
 9. The fuel cell system according to claim 1, wherein said controller controls said combustion fuel feeder so as to reduce the feed rate of the combustion fuel to said combustor in a stepwise fashion involving one or more steps, or in a continuous fashion.
 10. The fuel cell system according to claim 1, wherein said raw material is hydrocarbon gas.
 11. The fuel cell system according to claim 1, further comprising a raw material feeder configured to supply the raw material; wherein said controller allows said raw material feeder to supply the raw material to said fuel cell to fill the inside of said fuel cell with the raw material in shutdown operation or start-up operation.
 12. The fuel cell system according to claim 1, further comprising a gas concentration detector provided in said combustor or said off gas passage, for detecting a specified gas concentration; wherein said controller controls said combustion fuel feeder in response to an output signal from said gas concentration detector to reduce the feed rate of the combustion fuel to said combustor, after controlling said switching valve so as to switch from said bypass passage to said fuel cell to allow a supply of the fuel gas generated in said fuel gas generator to said fuel cell.
 13. The fuel cell system according to claim 12, wherein said controller controls said combustion fuel feeder so as to reduce the feed rate of the combustion fuel to said combustor, when said gas concentration detector detects an increase in raw material concentration.
 14. The fuel cell system according to claim 13, further comprising a flame-rod type flame sensor provided as said gas concentration detector in said combustor, wherein said raw material is a gas containing hydrocarbon, and wherein said controller controls said combustion fuel feeder to reduce the feed rate of the combustion fuel to said combustor, when said flame sensor detects an increase in the raw material concentration, after said controller controls said switching valve so as to switch from said bypass passage to said fuel cell to allow a supply of the fuel gas generated in said fuel gas generator to said fuel cell. 